Leg 183
will address four first-order problems related to the
characterization and quantification of maficigneous crustal
production and its effects during the Cretaceous and Cenozoic.
Our objectives are to

identify
and interpret relationships between LIP development and
tectonism.

Perhaps the
most significant question is how much magma was erupted over what
time interval? More specifically, (1) what time interval is
represented by the uppermost volcanic basement of this LIP? (2)
Do eruption ages vary systematically with location on the
plateau? (3) Was the growth episodic or continuous? (4) Did the
plateau grow by lateral accretion (i.e., similar to Iceland) or
by vertical accretion and underplating? Answers to these
questions, to be provisionally provided by dating the oldest
sediment above basaltic basement and later more definitively by 40Ar/39Ar
dating of the basalts, are required to understand the generation
of voluminous magma, the physical processes of magma intrusion
and extrusion, and to assess the impact of Cretaceous volcanism
on the surficial environment by estimating fluxes into the
ocean-atmosphere system. An aspect of oceanic plateau volcanism
that has been explored in only cursory detail (e.g., Sevigny et
al., 1992) is the role of hydrothermal alteration in controlling
elemental and isotopic fluxes. The extent, nature, and duration
of hydrothermal processes on the plateau can be determined by
drilling several holes with 150-200 m basement penetration.

a.
Composition (major and trace elements) and isotopic ratios
(Sr, Nd, and perhaps others) of unaltered phenocrysts
(typically olivine, plagioclase, and clinopyroxene). Such
data will provide information on parental magma composition
and the role of crustal processes such as fractional
crystallization, magma mixing, and assimilation.

b.
Composition (major and trace element) isotopic ratios (O, Sr,
Nd, Pb, Hf, and Os) of whole rocks. Different subsets of
these geochemical data will be used to understand both
magmatic and post-magmatic processes and the role of
geochemically distinct mantle and crustal components in these
rocks.

Several
lines of evidence support the interpretation that the Kerguelen
plume has been a long-term source of magma for major bathymetric
features in the eastern Indian Ocean. For example, the systematic
south-north age progression on Ninetyeast Ridge is consistent
with a hot spot track formed as the Indian plate migrated
northward over the Kerguelen plume (Mahoney et al., 1983; Duncan,
1991). Also, isotopic similarities between lavas from the
Ninetyeast Ridge, the younger lavas forming the Kerguelen
Archipelago and Heard Island, and the older lavas forming the
Kerguelen Plateau and Broken Ridge (Fig. 5B, C) indicate that the
Kerguelen plume played an important role (Weis et al., 1992; Frey
and Weis, 1995, 1996). The presence of a LIP, perhaps resulting
from decompression of a plume head, and an associated long-lived
hot-spot track present an excellent opportunity to understand a
long-lived mantle plume.

Many
studies of ocean island volcanoes have demonstrated that
geochemically distinct sources (e.g., the plume, entrained
mantle, and overlying lithosphere) contribute to plume-related
volcanism. Because isotopic characteristics of plume,
asthenosphere, and lithosphere sources are usually quite
different, temporal geochemical variations in stratigraphic
sequences of lavas can be used to determine the relative roles of
plume, asthenosphere, and lithosphere sources in plume-related
volcanism. Establishing how the proportion of these sources
changes with time and location provides an understanding of how
plumes "work" (e.g., Chen and Frey, 1985; Gautier et
al., 1990; White et al., 1993; Peng and Mahoney, 1995). A
continental lithosphere source component has also been recognized
in some lavas from the SKP and eastern Broken Ridge (Mahoney et
al., 1995). Also, wide-angle seismic data collected by
ocean-bottom seismometers in the Raggatt Basin of the SKP have
defined a reflective zone at the base of the crust, which has
been interpreted to be stretched continental lithosphere (Operto
and Charvis, 1995, 1996). The present geochemical data set shows
that a continental lithosphere component is obvious in lavas at
only two sites (dredge site 8 on Broken Ridge and Site 738 in the
SKP [Fig. 5B, C]). There is no
evidence for a continental component in lavas from the Central
Kerguelen Plateau, the Ninetyeast Ridge, and the Kerguelen
Archipelago (Frey et al., 1991; Yang et al., 1998). Determining
the spatial and temporal role of this lithosphere component is
required to evaluate whether this continental component is a
piece of Gondwana lithosphere that was incorporated into the
plume.

In addition
to answering questions about plume-lithosphere interactions,
geochemical data for plateau lavas will define the role of
depleted asthenosphere in creating this plateau. A MORB-related
asthenosphere is apparent in some of the Ninetyeast Ridge drill
sites (e.g., as reflected by the Sr and Nd isotopic ratios of
lavas from Site 756; Weis and Frey, 1991) and is an expected
consequence of the plume being close to a spreading ridge axis
during formation of the Ninetyeast Ridge. Relatively shallow
basement holes (150-200 m) in the Kerguelen Plateau can be used
to define spatial and short-term variability during the waning
phase of plateau volcanism. A surprising result of the shallow
penetrations of the Kerguelen Plateau is that sampling of 35-50 m
of igneous basement at several plateau sites shows that lavas at
each site have a suite of distinctive geochemical
characteristics: each site has a distinctive combination of Sr
and Nd isotopic ratios (Fig. 5B). Does this
heterogeneity reflect spatial heterogeneities in a plume or
localized differences in mixing proportions of components derived
from asthenosphere, plume, and slivers of continental
lithosphere? Interpretation requires knowledge of temporal
variations in geochemical characteristics at several locations.

Leg 183
will address the environmental impact of the formation of
Kerguelen and Broken Ridge. Important goals for this assessment
are to (1) define the post-magmatic compositional changes
resulting from interaction of magmas with the surficial
environment, (2) determine the relative roles of submarine and
subaerial volcanism in constructing the upper part of the
plateau, and (3) estimate volatile contents of magmas from
compositional studies of phenocrysts and their inclusions. The
study of altered and metamorphosed basement rocks will be a major
source for this information, but important information will also
be provided by overlying sediments. From these data, fluxes of
elements, including volatiles, particulates, and heat from
Kerguelen/Broken Ridge into the atmosphere-hydrosphere-biosphere
system, can be estimated and their environmental impact assessed.

To
understand the relationships between LIP magmatism and tectonic
events, we will study Kerguelen and Broken Ridge's seismic
volcanostratigraphy i.e., seismic facies analysis linked with
petrophysics and borehole data with various aspects integrated by
synthetic seismic modeling. We seek to determine stratigraphic
and structural relationships, both within the various Kerguelen
domains and Broken Ridge and between these features and adjacent
oceanic crust. Seismic volcanostratigraphic studies can reveal
temporal and spatial patterns of LIP extrusion in a regional
tectonic framework as well as test for synchronous or
asynchronous post-emplacement tectonism of the Kerguelen Plateau,
Broken Ridge, and adjacent ocean basins. Increased knowledge of
the vertical and tectonic histories of the Kerguelen Plateau and
Broken Ridge will provide insights into and much-needed boundary
conditions for models of mantle upwelling, crustal thinning,
lithospheric thermal histories, crustal growth histories, and
post-constructional subsidence and faulting.

Observations
of physical volcanology (e.g., flow thicknesses and directions,
morphology, vesicle distribution, presence and nature of
interbeds, and subaerial vs. submarine extrusion) will provide
important information on the distribution of melt conduits and
fluxes. Physical volcanology provides ground truth for seismic
volcanostratigraphy. We seek to understand how the uppermost
crust of Kerguelen and Broken Ridge formed, to locate surficial
or shallow subsurficial sources for the basalts (discrete
volcanoes or feeder dikes), to document environments of basalt
extrusion, and to assess effects of pre-existing bathymetry and
topography on flow distribution.